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from the thermal conductivity which is approximately 40 times higher than that of talc. This might be related to the poor interconnectivity of the particles in the composite, which was shown by Weidenfeller et al. [3]. It was shown that the interconnectivity, which is a relative measure to an ideally interconnected network of high conductivity par- ticles, is for copper in a polypropylene matrix lower than 1% and very poor compared to interconnectivity of magnetite with 55% or the interconnectivity of barite with 46% [3]. The authors also discussed influences of particle size and shape on the interconnectivity in a polypropylene matrix [2,3].
The necessary time to cool down the surface of the composite in the cavity to 60 8C is shown in Fig. 7.
Fig. 7. Dependence of cooling time (from 200 down to 60 8C) from filler fraction for various polypropylene matrix composites. The symbols are measured values, the lines represent linear fits.
B. Weidenfeller et al. / Composites: Part A 36 (2005) 345–351 351
Table 6
Regression lines of the cooling time (s) versus filler fraction (vol%) shown in Fig. 7
Composite Slope (K/s) t (from 200 to 60 8C)
(s)
PP – 50.5
PPCFe3O4 K0.340 40.5
PPCBaSO4 K0.322 40.7
PPCCu K0.600 33.8
PPCglass fibres K0.290 41.8
PPCtalc K0.280 42.5
PPCSrFe12O19 K0.322 40.9
Cooling time t represents the time span to cool down a polypropylene-filler composite with 30 vol% filler in the cavity of a mould from a mass temperature of TMZ200 down to 60 8C (333.15 K).
The cooling time is linearly dependent on the filler volume fraction in the polypropylene matrix. The data of the calculated regression lines are listed in Table 6. It can be clearly seen that the copper filled polypropylene cools down much faster than the other investigated composites. The cooling behaviour of polypropylene with barite, strontium ferrite and magnetite is similar, whereas the magnetite cools down a little bit faster than all other materials.
5. Conclusions
The cooling behaviour of polypropylene in the injection moulding process can be reduced by the used magnetite, barite, strontium ferrite, glass fibre, talc and copper fillers. The cooling behaviour cannot solely be explained by a simple exponential law derived from the Fourier’s law of heat conduction, due to the temperature dependence of the heat transfer and latent heat during solidification. Further- more, the cooling curves show different merging sections, which are affected by the after pressure at high temperatures and low times in the injection moulding cycle, thermal diffusivity of the composite at medium times of the injection moulding cycle and the thermal diffusivity of the poly- propylene matrix at the end of the injection moulding cycle. Additionally, an anisotropy of the thermal conductivity, e.g. for talc filler, or a low interconnectivity of particles, e.g. copper, influences the cooling behaviour.
For the used materials and in the investigated range of filler fractions the cooling time for cooling down the injection moulded composite from a temperature of 200 down to 60 8C is linearly dependent on the filler fraction. For 35 vol% copper in the polypropylene matrix the cooling time could be reduced from 50.5 (unfilled PP) to